The gasification of coal by means of an economic and efficient technology has been a focus of attention for many years. Spurred by the increasing energy shortage, it has provided the motive for the introduction of a variety of second generation gasification schemes which are presently in varying stages of development. These are based on one of three gasification approaches, namely, fixed bed, fluidized bed, or entrained gasifiers. These processes are handicapped by all, or nearly all, of the following requirements:
1. Need for an oxygen plant. PA1 2. Product desulfurization required. PA1 3. Excessive amounts of cooling water required. PA1 4. Incomplete carbon utilization. PA1 5. Limited to non-caking coals, or PA1 6. Pre-treatment of caking coals required. PA1 7. Tars produced and must be eliminated. PA1 8. Expensive refractories required. PA1 9. Large amounts of heating fuel required. PA1 10. High pressure for reaction vessles may be required. PA1 11. Low gasification rates. PA1 12. Expensive methanation catalysts used.
Improvements over the traditional gasification processes, such as the Lurgi or Koppers-Totzek processes, appear to be marginal at best. The net result is that the cost of product gas by any of these second generation processes is prohibitive and, more particularly, the capital investment projected for commercial-size plants has been so large as to inhibit their construction.
It has been proposed in U.S. Pat. No. 4,080,550 to inject powdered coal in a carrier gas into a free-burning arc column. Likewise, it was proposed in U.S. Pat. No. 3,644,781 to inject powdered coal in hydrogen into a free-flowing arc column.
Methods and devices for transferring energy to fluid materials also by exposing said fluid material to the energy of a high intensity arc have been previously reported. For example, in U.S. Pat. No. 3,209,193, a novel method of exposing the fluid to the energy of the arc is disclosed, which consists of passing the fluid continuously through a porous anode so that it enters the discharge via the active anode surface, i.e., where said surface is acting as the arc terminus. That patent further discloses that unique and valuable results can be obtained if certain criteria are satisfied in operating such a device.
U.S. Pat. No. 3,214,623 describes an improvement to the above patent where the arc discharge has an essentially conical geometry. The cathode, porous anode and insulating supports are arranged geometrically to each other, so that the conduction column assumes the shape of an axially symmetrical conical shell.
The technique of fluid injection through a porous anode has been termed the "fluid transpiration arc" (FTA), and is an example of the use of a high intensity arc to transfer energy to materials.
Attempts have also been made to inject a working fluid into the interior of an arc column at other points than the anode. Many difficulties have been found in these attempts. For example, in the constricted arc column of a conventional wall-stabilized arc with a segmented, watercooled constrictor channel long enough to assure the establishment of a fully developed column, the injected gas is forced to flow axially, concentric and parallel to the conduction column. Since the column in this device is subject to an appreciable thermal constriction, it would seem that the convected gas would be forced through the column boundary into the primary energy dissipating zone. It was found, however, that, even in the fully developed region, beyond which the radial distributions of the flow parameters remain essentially constant, by far the major part of the flow traverses the thin, cool, nonconducting gas film adjacent to the channel wall. In fact only about 10 percent of the mass flow enters the hot core of the constructed arc column. The much higher density and lower viscosity of the cool gas in the wall layer, plus the fact that even a very thin film can have appreciable cross-sectional area near the wall, compensate for the lower velocity of the cool gas layer, and account for nearly all of the convected mass flow. It should be noted that the radial temperature across the fully developed portion of the column remains above 10,000.degree. K., over 80 percent of the channel diameter, so that the plasma fills the channel quite well. The conclusion is that most of the working fluid does not penetrate the column and is therefore not directly exposed to the zone of maximum energy dissipation.
The same effect is noted with other flow configurations. For example, if a stream of gas is projected at right angles against the column of a free-burning arc, the arc will be blown out at quite low flow rates. However, the column can be stabilized by a magnetic field of suitable strength oriented normal to both column and gas flow so as to balance exactly the force of convection. Even when the balance is established at very high-flow rates, the gas does not enter the column, but is deflected around it, the column behaving much like a hot solid cylinder. An examination of existing arc jet devices reveals that in nearly every case most of the working fluid does not penetrate into the column and is not subjected to the zone of direct energy transfer.
A most important development in the process of injecting a working fluid into the interior of an arc column was described in U.S. Pat. Nos. 3,644,781 and 3,644,782. These patents describe how the contraction zone, wherein the current-carrying area of the arc column decreases and which is formed adjacent to the cathode tip, can serve as an "injection window" into the arc column. Thus, when a gas is caused to impinge directly on the contraction zone boundary it will penetrate into the arc column at flow rates far in excess of what can be forced across the cylindrical column boundary of the arc. Gas flow rates of the magnitude much greater than that aspirated naturally can be injected into the column without disturbing the stability of the arc provided the gas is forced to follow the conical configuration of the cathode tip and impinges on the column at the contraction zone. For this purpose, the gas to be injected must be formed in a high-velocity layer and projected along the conical cathode surface.
By proper adjustment of the gas velocity and cone angle of the cathode, the gas can be made to cross the column boundary in essentially the same general direction as would the aspirated ambient gas stream in the absence of forced cnvection. The optimum cone angle for this purpose appears to be between 30.degree. and 60.degree..
A second critical parameter described in these patents is the injection velocity. This can be be varied without altering the total mass flow (convection rate) by varying the area of the annular orifice and changing the inlet gas pressure as required to maintain a fixed flow rate. It was observed, for example, that as the injection velocity (mass flow density) was varied, the column temperature passes through a peak, with the maximum temperature rising to two or three times that obtained when the velocity was several times higher or lower than its optimum value.
A third critical parameter described in these patents is the total mass flow of the injected fluid medium. As the total mass flow of the injected fluid medium is varied at substantially constant current levels and mass flow density, an alteration of the shape of the contraction zone occurs. When the total mass flow or convection rate of the injected fluid medium is increased from zero, little or no change in the shape of the contraction zone is observed and substantially all of the injected fluid enters the arc column through the injection window. However, as the total mass flow of the injected fluid medium is increased further, at a point depending on the medium injected, the contraction zone begins to elongate, thus decreasing the space rate of contraction of the arc column diameter. This space rate of contraction may be characterized by the window angle .alpha. (see FIG. 1). When the angle .alpha. is sufficiently reduced, that is, about 40.degree. or less, the major portion of the flow of the fluid medium does not enter the arc column.
This technique of injecting the working fluid into the contraction region of the arc column has been termed the "forced convection cathode" are (FCC), and is principally described in U.S. Pat. No. 3,644,782. U.S. Pat. No. 3,644,781 describes the operation of the FCC with a heterogeneous material where the introduction into the fluid medium injected of a finely divided non-gaseous material causes an enlargement of the window angle .alpha., thus enabling the insertion of an increased amount of the non-gaseous material.
An improvement in the operation of the FCC is described in U.S. Pat. No. 3,900,762 which involves interposing a stream of shielding gas between the cathode producing the arc and the reactive material being inserted into the arc.
A further improvement in the operation of the FCC with insertion of large amounts of reactive material such as powdered coal is described in U.S. Pat. No. 4,080,550 which describes an improvement in the process of energizing a reactive material comprising a solids-containing fluid medium by means of a free-burning arc discharge between an anode and a cathode having a conical tip, wherein said arc discharge forms a contraction of the current-carrying area in the transition region adjacent to the cathode and wherein said reactive material is forcefully projected along the surface of said conical tip of said cathode into and through said contraction of the current-carrying area in the transition region adjacent to the cathode, the said improvement comprising projecting said reactive material through a plurality of individual linear feed channels having a constant flow cross-sectional area, said individual feed channels being supplied from a common source of a solids-containing fluid medium through flow splitters having two or more converging channels on the outlet side of equal cross-sectional area forming an angle of 15.degree. or less opening into a channel of the same or greater cross-sectional area as the combined area of the two or more converging channels whereby the flow of said solids-containing fluid medium is divided into streams of equal flow rate and grain loading; and extensively cooling the outlet area of said plurality of individual linear feed channels whereby the surace temperature of said outlet area is maintained below the temperature at which the solids in said solids-containing fluid medium agglomerate.
When the concept is applied to the treatment of powdered coal and steam in the plasma arc, the cost of electrical energy is about three times that of the equivalent amount of the thermal energy generated by the combustion of fossil fuels. This is especially true when the electrical heating source involves an arc plasma device which generates temperatures an order of magnitude greater than required for coal gasification and which, therefore, is subject to greater losses than, for instance, an electrical resistance furnace.
The first step in most gasifiers is the well-known water-gas reaction: EQU C+H.sub.2 O.fwdarw.H.sub.2 +CO (1)
This reaction is quite endothermic. Thus, assuming the product gases issue at 1000.degree. K., a calculation of the heat input to supply reaction (1) plus sensible heat, totals 1232 K-Cal per lb. of coal fed, based on Sewickley coal obtained from southwestern Pennsylvania. In electrical units this amounts to 1.43 KWH/lb. of coal. To supply all of this energy electrically would be very expensive, especially in consideration of the low efficiency of electrical power generation.
Accordingly, the process of the present invention is so operated as to reduce the consumption of electrical energy to a minimum, i.e., to a small fraction of the 1.43 KWH per lb. of coal cited above, preferably considerably less than 0.5 KWH/lb. of coal. The manner in which this is accomplished by a two-stage treatment of the coal will be explained in detail in the following.